1. Field of the Invention
The present invention relates to a boundary microphone having a membrane pressure-sensitive switch that turns on or off an output signal of a microphone unit and mainly installed on a desk for use.
2. Related Background Art
Some boundary microphones have membrane pressure-sensitive switches that turn on or off output signals of microphone units and are mainly installed on desks for use. A boundary microphone is also referred to as a surface mount microphone, as disclosed in Japanese Unexamined Patent Application Publication No. 2008-288933, for example, since the boundary microphone is installed and used on a desk or a floor in a TV studio or a conference room. The boundary microphone, which is mainly used on a desk as disclosed in Patent Literature 1, for example, often has a low-profile flat case in which a microphone unit and necessary circuits are installed.
With reference to FIG. 6, a boundary microphone 11 primarily includes a flat metal base 10 having an open upper surface, a metal microphone cover 15 having numerous openings (sound wave inlets) and mounted on the base 10 so as to cover the upper surface of the base 10, a membrane pressure-sensitive switch 1, a male screw 12, a circuit board 18 of the boundary microphone 11, and a microphone unit 13. A microphone cord 16 and a cord bush 17 are provided in the rear portion (right end in FIG. 6) of the base 10.
The boundary microphone 11 may have a switch section to allow a user to turn on or off the output signal of the microphone unit 13, the switch section including a push switch of any type, such as a membrane, capacitance, or mechanical switch. A click-on/off pushbutton switch generates vibration on operation thereof, thus vibrating a microphone main body and causing vibration noise. As shown in FIG. 6, the boundary microphone 11 thus employs the membrane pressure-sensitive switch 1 to turn on or off the output signal since the operation noise hardly impacts audio signals during operation of the microphone.
With reference to FIG. 7, the membrane pressure-sensitive switch 1 is generally composed of a membrane 4 that yields to pressure of a user, a circuit board 2 provided with an electrode pattern 5 that detects electrical conductivity, and a spacer 3 interposed between the membrane 4 and the circuit board 2. The membrane 4 is pressed to come in contact with the electrode pattern 5, and thus to turn on the membrane pressure-sensitive switch 1. The membrane 4 is released to turn off the membrane pressure-sensitive switch 1. However, the electrode pattern 5 that detects conductivity with a copper foil is disposed on the circuit board 2 of the membrane pressure-sensitive switch 1 so as to face the membrane 4. The electrode pattern 5 is composed of two interdigital electrodes, as shown in FIG. 8, i.e., an electrode 5A and another electrode 5B. Thus, the electrode pattern 5, which is not generally connected with the copper foil in an off state, as shown in FIG. 7, is exposed to the space defined by the membrane 4, the circuit board 2, and the spacer 3. The electrode pattern 5 thus exhibits an effect similar to an antenna and is very vulnerable to external electromagnetic waves. Furthermore, the spacer 3 is generally composed of an organic resin material, which allows electromagnetic waves to permeate. Accordingly, the boundary microphone having such a configuration is readily affected by electromagnetic waves from cellular phones and other device, and thus may malfunction or generate noise.